Rear driver door doesn't lock

Hello there, once again I've run into a lil problem that's not so much bad but rather annoying to me . my problem is that the rear drivers door will not lock with with key fob or switch on doors. I've checked the fuses. What could it be?

Okay just finished taking apart the door disconnected and reconnected the three wires going into Motorola black box. And also I'm guessing the solenoid is the actual part that controls the lock? I wd-40 it cause it was sticking and it helped a lil but its wearing out.